The present invention is concerned with an improved distillative process for removing glycerine from organic compounds of nearly equivalent boiling point. More specifically, the invention is concerned with the removal of substantial quantities of glycerine from mixtures comprising glycerine and 4-hydroxymethyl-1,3-dioxanes, 5-hydroxy-1,3-dioxolanes and mixtures thereof.
The preparation of certain 4-hydroxymethyl-1,3-dioxanes and 5-hydroxy-1,3-dioxolanes by reaction under transesterification conditions between glycerine and certain acetals and ketals has been described in U.S. patent application Ser. No. 159,332, filed June 16, 1980, now U.S. Pat. No. 4,320,024. The products formed in the reaction have been found to be usefully employed in various hydraulic fluid formulations. A preferred reaction mixture is formed by the reaction of glycerine and 4,4-dialkoxybutanenitriles, alternatively the same reaction product may be formed by reaction of glycerine directly with the corresponding carbonyl compound, e.g., 4-oxobutanenitrile. For its teaching of the above reaction and the conditions thereof, the above-identified patent application is hereby incorporated by reference.
The presence of glycerine, especially in amounts greater than about 1.0 percent by weight of the acetal and ketal reaction products detrimentally affects the properties of the product particularly in such applications as hydraulic fluids where high wet boiling points of the fluids are adversely affected by the presence of even small amounts of glycerine.
In the above-described transesterification reaction, it has been found that the initial product mixture may contain small amounts of unreacted glycerine. In fact, amounts on a weight basis from about 2 percent to about 10 percent may be present. This glycerine contaminant may be removed by standard techniques of distillation, however, the process has proven particularly tedious and difficult due primarily to the high viscosity of the glycerine component, thereby generally rendering the use of modern packed distillation columns ineffective. As a result, the extended distillation times and elevated temperatures required for the process result in the formation of increased levels of tarry by-products and correspondingly reduced levels of the desired compounds and economic loss.
If an excess of the acetal or ketal reactant is employed in an attempt to drive the reaction to completion and consume the residual glycerine, a significant portion reacts in addition with the free hydroxyl group in the glycerine acetal or ketal reaction product thereby detrimentally forming higher molecular weight non-distilled residues which lower the overall yield.
It would be desirable to provide an improved process for the separative distillation of mixtures comprising glycerine, 4-hydroxymethyl-1,3-dioxanes and 5-hydroxy-1,3-dioxolanes.